1. Field of the Invention
This invention relates to high hardness and high toughness nitrogen-containing sintered alloys or cermets useful for cutting tools, in particular, high speed cutting, and processes for the production of the same.
2. Description of the Prior Art
Lately, nitrogen-containing sintered hard alloys (cermets) each comprising a hard phase containing titanium carbonitride as a predominant component bonded with a binder phase of nickel and/or cobalt have been put to practical use as cutting tools.
These nitrogen-containing sintered hard alloys have been used with cemented carbides even in the field of cutting tools or cutters to which it is next impossible to apply no nitrogen-containing sintered hard alloys of the prior art, since in these nitrogen-containing sintered hard alloys, the hard phase is of much finer grains and accordingly, the high temperature creeping resistance is much more improved, as compared with the no nitrogen-containing sintered hard alloys comprising a hard phase of carbides of titanium, etc. of the prior art.
However, the nitrogen-containing sintered hard alloys of the prior art are mainly of (Ti, Ta, W, Mo)(CN).Ni-Co types, in which molybdenum (Mo) is regarded as an indispensable component, because molybdenum, existing in an intermediate phase between a hard phase and binder phase, is capable of protecting the hard phase from the liquid phase during sintering and controlling the grain growth of the hard phase due to dissolving and precipitating. Since the nitrogen-containing sintered hard alloys of the prior art have such a tendency that the carbonitrides contained therein are decomposed when heated in vacuum during the process for the production thereof to retain pores after sintered, the strength thereof is often less than that of the cemented carbides of the prior art. This tendency becomes more remarkable the more is the content of nitrogen. In order to prevent the carbonitrides from decomposition, it has been proposed to improve the sintering method, for example, by effecting the sintering in a nitrogen atmosphere, but the improvement of the properties is not sufficient because of, for example, segregation tendency of the nitrogen contained therein.
The above described sintered hard alloys or cermets comprising hard dispersed phases of mixed carbonitrides of titanium (Ti), tantalum (Ta), molybdenum (Mo), tungsten (W), etc., bonded with heat resisting metals such as nickel (Ni) or cobalt (Co) are favorably compared with the sintered hard alloys or cemented carbides comprising hard phases of carbides of W, Ti, Ta, etc., bonded with metals such as Co with respect to the adhesion resistance on workpieces, and thus have widely been used as a material for high speed cutting tools. However, these cermets are so hard, similarly to the cemented carbides, that the grinding machinability is bad and grinding is impossible except using diamond wheels.
Furthermore, in comparison with the cemented carbides comprising hard phases of mixed carbides of W, Ti, Ta, etc., bonded with metals such as Ni or Co according to the prior art, the above described cermets are markedly more improved in thermal fatigue resistance and toughness, so the use thereof is being enlarged to the field in which only the cemented carbides comprising tungsten carbide as a predominant component can be used.
Of late, high speed cutting has more and more been desired in the field of cutting tools, but the nitrogen-containing sintered hard alloys have the disadvantages that the crater depth occurring on the rake face of a cutting tool proceeds very rapidly in high speed cutting. In this case, the crater depth means such a phenomenon that the hard phase of a nitrogen-containing sintered hard alloy is dug out with granular unit and then allowed to fall off. In general, the crater depth can be controlled by roughening the structure of an alloy, but this controlling method is naturally limited since the hardness is lowered as the structure is roughened.
For the production of the above described cermets, a method has hitherto been employed comprising mixing powdered titanium carbonitride and powdered carbides of molybdenum, etc., pressing and forming and then sintering. Increase of the nitrogen content in the hard disperse phase has lately been carried out so as to improve the cutting property of the cermets, but a denitrification phenomenon becomes vigorous with the increase of the nitrogen content, thus lowering the sintering property. Thus, addition of a large amount of Mo is indispensable for maintaining the sintering property and the grinding machinability of the cermets becomes worse.
When using the cermets as cutting tools, in particular, these can preferably be used for finishing which needs a high surface precision, because of the good deposition resistance. Accordingly, a throwaway insert of the so-called G grade (JIS G grade precision), obtained ordinarily by subjecting a cermet tool to grinding or machining, has been used from the standpoint of the precision of a finished surface or finished dimension of a workpiece. However, since the large nitrogen content cermets which have lately been developed are very hard to be machined even by the use of a diamond wheel, these have not been put to practical use but as only M-grade throwaway inserts which are not subjected to machining as sintered and in spite of the rapid progress of the cutting property, demands for the cermets are not increasing.
In the above described cermets, the properties such as wear resistance, toughness, etc. depend largely on the composition of the hard phase, in particular, the ratio of non-metallic elements to alloyed metallic elements, as well known in the art. For example, in a cermet comprising a hard dispersed phase represented by the general formula (Ti, M')(C,N).sub.m wherein M' is a transition metal such as Nb, Ta, Mo or W, bonded with a metal such as Ni or Co, it is known that the hardness of the cermet is monotonously increased with the increase of m, that is, the larger the value of m, the larger the hardness. Therefore, it is needless to say that m is maintained as large as possible from the standpoint of the most important wear resistance for cutting tools.
On the other hand, it is known that the equillibrium nitrogen partial pressure of (Ti, M')(CN).sub.m is monotonously decreased with the decrease of m, that is, the smaller the value of m, the lower the equilibrium nitrogen partial pressure. When the equilibrium nitrogen partial pressure of the hard phase is higher, there takes place such a denitrification phenomenon that nitrogen gets out of the sintered compact during sintering and the thus resulting cermet is not homogeneous to warp the surfaces and sides thereof and does not satisfy the standard as a M-grade throwaway insert, since not only the nitrogen content does not reach a predetermined amount, but also the denitrification does not proceed homogeneously. From the above described reasons, the value of m must be adjusted to at most 0.80.